The pilots of the runaway Learjet had as little as 10 seconds to react after an explosive loss of cabin pressure, aviation experts said yesterday.

If they failed to put on oxygen masks in time, hypoxia would have knocked them unconscious before they could bring the plane down to a safe altitude.

Frost on the Learjet’s windshield is the biggest tip-off the plane experienced a pressure drop – humidity inside the cabin froze as temperatures plummeted to far below zero.

If the change came slowly – such as when an engine valve that forces warm air into the cabin malfunctions – the crew should have had enough warning to land the aircraft.

In this case, chances are the jet was rocked by an “explosive decompression” – a massive, sudden loss of pressure.

“It could be a windshield or window popping out, a cargo door or entrance door loose or you could have a crack in a pressure bulkhead,” said Bob Vandel, executive vice president of the Flight Safety Foundation.

Alarms and flashing indicator lights would have alerted the crew to the catastrophe – and they would have to act fast.

“Once you lose pressure, you have a very limited time of consciousness – some people say 10 to 15 seconds, others say 30 seconds,” he said.

Decompression at 40,000 feet would have two effects – less oxygen in the air and not enough pressure to force what little oxygen there is into the lungs and bloodstream.

To stay alive, the pilots and passengers would need to put on oxygen masks, which are supposed to drop down as soon as the pressure plunges.

The pilots would then have to put the plane into an “emergency dive-down” and get it to 10,000 feet or lower – the altitude at which the body can properly absorb oxygen.

“The pilots’ oxygen masks are sophisticated and they should have been able to control the plane to an emergency descent and then land it right away,” said David Almy, vice president of strategic programs for the National Business Aviation Association.

“So the question, and it’s a complex question, is: Why didn’t the crew take control of the aircraft?”

On the mechanical end, investigators will have to determine if the oxygen masks worked correctly. If they did, human error could have been the deciding factor.

“If you were confused or tired or fatigued that could certainly slow your reaction time,” Vandel notes.

Whatever the cause, the experts said that explosive decompression is a situation all pilots prepare for but few ever face.

“We have been talking about the number of times aircraft have gone down where the crew and passengers have been incapacitated,” Almy said. “And we can only remember two in the last 40 years.”